Optical fiber waveguides, which are able to guide light by virtue of a so-called photonic bandgap (PBG), were first proposed in 1995.
In, for example, “Full 2-D photonic bandgaps in silica/air structures”, Birks et al., Electronics Letters, 26 Oct. 1995, Vol. 31, No. 22, pp. 1941-1942, it was proposed that a PBG may be created in an optical fiber by providing a dielectric cladding structure, which has a refractive index that varies periodically between high and low index regions, and a core defect in the cladding structure in the form of a hollow core. In the proposed cladding structure, periodicity was provided by an array of air holes that extended through a silica glass matrix material to provide a PBG structure through which certain wavelengths and propagation constants of light could not pass. It was proposed that light coupled into the hollow core defect would be unable to escape into the cladding due to the PBG and, thus, the light would remain localized in the core defect.
It was appreciated that light travelling through a hollow core defect, for example filled with air or even under vacuum, would suffer significantly less from undesirable effects, such as non-linearity and loss, compared with light travelling through a solid silica or doped silica fiber core. As such, it was appreciated that a PBG fiber may find application as a transmission fiber to transmit light over extremely long distances, for example across the Atlantic Ocean, without undergoing signal regeneration, or as a high optical power delivery waveguide. In contrast, for standard index-guiding, single mode optical fiber, signal regeneration is typically required approximately every 80 kilometers.
The first hollow core PBG fibers that were attempted by the inventors had a periodic cladding structure formed by a triangular array of circular air holes embedded in a solid silica matrix and surrounding a central air core defect. Such fibers were formed by stacking circular or hexagonal capillary tubes, incorporating a core defect into the cladding by omitting a central capillary of the stack, and then heating and drawing the stack, in a one or two step process, to form a fiber having the required structure. The first fibers made by this process had a core defect formed by the omission of a single capillary from the center of the cladding structure.
U.S. Pat. No. 6,404,966 describes what is stated therein to be a PBG fiber having a hollow core region, which has an area of several times the optical wavelength, and a PBG cladding having a pitch equal to half the optical wavelength. The suggested advantage of the fiber is that it exhibits single mode behaviour.
International patent application PCT/GB00/01249 (The Secretary of State for Defence, UK), filed on 21 Mar. 2000, proposed the first PBG fiber to have a so-called seven-cell core defect, surrounded by a cladding comprising a triangular array of air holes embedded in an all-silica matrix. The core defect was formed by omitting an inner capillary and, in addition, the six capillaries surrounding the inner capillary. This fiber structure appeared to guide one or two modes in the core defect, in contrast to the previous, single-cell core defect fiber, which appeared not to support any guided modes in the core defect.
According to PCT/GB00/01249, it appeared that the single-cell core defect fiber, by analogy to the density-of-states calculations in solid-state physics, would only support approximately 0.23 modes. That is, it was not surprising that the single-cell core defect fiber appeared to support no guided modes in its core defect. In contrast, based on the seven-fold increase in core defect area (increasing the core defect radius by a factor of √7), the seven-cell core defect fiber was predicted to support approximately 1.61 spatial modes in the core defect. This prediction was consistent with the finding that the seven-cell core defect fiber did indeed appear to support at least one guided mode in its core defect.
A preferred fiber in PCT/GB00/01249 was described as having a core defect diameter of around 15 μm and an air-filling fraction (AFF)—that is, the proportion by volume of air in the cladding—of greater than 15% and, preferably, greater than 30%. Herein, AFF (or any equivalent measure for air or vacuum or other materials) is intended to mean the fraction by volume of air in a microstructured, or holey, portion of the cladding, which is representative of a substantially perfect and unbounded cladding. That is, imperfect regions of the cladding, for example near to or abutting a core defect and at an outer periphery of a microstructured region, would not be used in calculating the AFF. Likewise, a calculation of AFF does not take into account over-cladding or jacketing layers, which may surround the microstructured region.
In “Analysis of air-guiding photonic bandgap fibers”, Optics Letters, Vol. 25, No. 2, Jan. 15, 2000, Broeng et al. provided a theoretical analysis of PBG fibers. For a fiber with a seven-cell core defect and a cladding comprising a triangular array of near-circular holes, providing an AFF of around 70%, the structure was shown to support one or two air guided modes in the core defect. This agreed with the finding in PCT/GB00/01249.
In the book chapter entitled “Photonic Crystal Fibers: Effective Index and Band-Gap Guidance” from the book “Photonic Crystal and Light Localization in the 21st Century”, C. M. Soukoulis (ed.), ©2001 Kluwer Academic Publishers, the authors presented further analysis of PBG fibers based primarily on a seven-cell core defect fiber. The optical fiber was fabricated by stacking and drawing hexagonal silica capillary tubes. The authors suggested that a core defect must be large enough to support at least one guided mode but that, as in conventional fibers, increasing the core defect size would lead to the appearance of higher order modes. This statement appears to contradict the position presented in the aforementioned U.S. Pat. No. 6,404,966, which prescribes a large core region and single mode behaviour. The authors of the chapter also went on to suggest that there are many parameters that can have a considerable influence on the performance of bandgap fibers: choice of cladding lattice, lattice spacing, index filling fraction, choice of materials, size and shape of core defect, and structural uniformity (both in-plane and along the axis of propagation).
WO 02/075392 (Corning, Inc.) identifies a general relationship in PBG fibers between the number of so-called surface modes that exist at the boundary between the cladding and core defect of a PBG fiber and the ratio of the radial size of the core defect and a pitch of the cladding structure, where pitch is the center to center spacing of nearest neighbor holes in the triangular array of the exemplified cladding structure. It is suggested that when the core defect boundary, together with the photonic bandgap crystal pitch, are such that surface modes are excited or supported, a large fraction of the “light power” propagated along the fiber is essentially not located in the core defect. Accordingly, while surface states exist, the suggestion was that the distribution of light power is not effective to realize the benefits associated with the low refractive index core defect of a PBG crystal optical waveguide. The mode energy fraction in the core defect of the PBG fiber was shown to vary with increasing ratio of core defect size to pitch. In other words, it was suggested that the way to increase mode energy fraction in the core defect is by decreasing the number of surface modes, in turn, by selecting an appropriate ratio of the radial size of the core defect and a pitch of the cladding structure. In particular, WO 02/075392 states that, for a circular core structure, a ratio of core radius to pitch of around 1.07 to 1.08 provides a high mode power fraction of not less than 0.9 and is single mode. Other structures are considered, for example in FIG. 7 (of WO 02/075392), wherein the core defect covers an area equivalent to 16 cladding holes.
The reason why varying the ratio of the radial size of the core defect and a pitch of the cladding structure affects the nature of the surface modes supported by a PBG fiber can be explained with reference to the book “Photonic Crystals: Molding the Flow of Light”, Joannopoulos et al., Princeton University Press, ISBN 0-691-03744. The text describes in detail the nature of surface modes and, in particular, the reasons why they form at an interface between a PBG structure and a defect (or other termination of the PBG structure). In brief, surface modes occur when there are electromagnetic modes near the surface, but they are not permitted to extend into the PBG crystal at the respective frequency due to the PBG. The book goes on to describe that the characteristics, and indeed the presence at all, of the surface modes can be tuned by varying the termination position of the PBG structure. For example, a PBG structure that terminates by cutting through air holes has different surface mode characteristics than the same PBG structure that terminates by cutting through only solid material around holes. WO 02/075392 is consistent with this since varying the core defect size of a PBG fiber naturally varies the termination position of the PBG structure.
In a Post-deadline paper presented at ECOC 2002, “Low Loss (13 dB) Air core defect Photonic Bandgap Fiber”, N. Venkataraman et al. reported a PBG fiber having a seven-cell core defect that exhibited loss as low as 13 dB/km at 1500 nm over a fiber length of one hundred meters. The structure of this fiber closely matches the structure considered in the aforementioned book chapter. The authors attribute the relatively small loss of the fiber as being due to the high degree of structural uniformity along the length of the fiber.
The present applicant's post-deadline paper presented at OFC 2004, “Low loss (1.7 dB/km) hollow core photonic bandgap fiber”, Mangan et al, reported the lowest ever reported loss for a PBG fiber, which had a nineteen cell core defect and exhibited loss below 2 dB/km at 1565 nm, and goes on to propose that scaling the fiber to operate at a longer wavelength should reduce loss even further. In conventional state-of-the-art solid silica fibers, attenuation is dominated by Rayleigh scattering and multi-phonon absorption at short and long wavelengths, respectively, resulting in an attenuation minimum at around 1550 nm. In hollow-core PBG fibers most of the light does not travel in glass, and therefore the effects of Rayleigh scattering and multi-phonon absorption in the bulk material are significantly reduced, while the internal surfaces of the fiber become a potentially much more important contributor to loss. Theoretical considerations indicate that the attenuation due to mode coupling and scattering at the internal air/glass interfaces, which dominate the loss in the fiber reported, should scale with the wavelength λ as λ-3. This was confirmed by the empirical data showing the minimum loss of hollow-core PBG fibers designed for various operating wavelengths in a wavelength range where IR absorption is negligible. It is likely that silica hollow-core PBG fibers will achieve their lowest loss somewhere in the 1800-2000 nm wavelength range, well beyond the wavelength at which bulk silica assumes its minimum loss.
An alternative kind of PBG fiber, which does not have a cladding comprising a lattice of high and low refractive index regions, is described in WO00/22466. These PBG fibers typically comprise, in a transverse cross section, concentric, increasingly large, annuli of varying high and low refractive index material, which create an omni-directional reflector capable of confining light to a core region of the fiber.
PBG fiber structures are typically fabricated by first forming a pre-form and then heating and drawing an optical fiber from that pre-form in a fiber-drawing tower. It is known either to form a pre-form by stacking capillaries and fusing the capillaries into the appropriate configuration of pre-form, or to use extrusion.
For example, in PCT/GB00/01249, identified above, a seven-cell core defect pre-form structure was formed by omitting from a stack of capillaries an inner capillary and, in addition, the six capillaries surrounding the inner capillary. The capillaries around the core defect boundary in the stack were supported during formation of the pre-form by inserting truncated capillaries, which did not meet in the middle of the stack, at both ends of the capillary stack. The stack was then heated in order to fuse the capillaries together into a pre-form suitable for drawing into an optical fiber. Clearly, only the fiber drawn from the central portion of the stack, with the missing inner seven capillaries, was suitable for use as a hollow core defect fiber.
U.S. Pat. No. 6,444,133 (Corning, Inc.), describes a technique of forming a PBG fiber pre-form comprising a stack of hexagonal capillaries in which the inner capillary is missing, thus forming a core defect of the eventual PBG fiber structure that has flat inner surfaces. In contrast, the holes in the capillaries are round. U.S. Pat. No. 6,444,133 proposes that, by etching the entire pre-form, the flat surfaces of the core defect dissolve away more quickly than the curved surfaces of the outer capillaries. The effect of etching is that the edges of the capillaries that are next to the void fully dissolve, while the remaining capillaries simply experience an increase in hole-diameter. Overall, the resulting pre-form has a greater fraction of air in the cladding structure and a core defect that is closer to a seven-cell core defect than a single cell core defect.
PCT patent application number WO 02/084347 (Corning, Inc.) describes a method of making a pre-form comprising a stack of hexagonal capillaries of which the inner capillaries are preferentially etched by exposure to an etching agent. Each capillary has a hexagonal outer boundary and a circular inner boundary. The result of the etching step is that the centers of the edges of the hexagonal capillaries around the central region dissolve more quickly than the corners, thereby causing formation of a core defect. In some embodiments, the circular holes are offset in the inner hexagonal capillaries of the stack so that each capillary has a wall that is thinner than its opposite wall. These capillaries are arranged in the stack so that their thinner walls point towards the center of the structure. An etching step, in effect, preferentially etches the thinner walls first, thereby forming a seven-cell core defect.